The major objective of the proposed research is to study protein degradation during development of the mouse ovum and preimplantation embryo. Although considerable work has been done on protein synthesis during this period, almost nothing is known about degradation or protein stability which is of equal importance to synthesis for control of protein concentration and cellular function. We have selected three important transition points in development for careful examination. These are (1) blastocyst formation (the differentiation into inner cell mass and trophoblast cells), (2) fertilization, and (3) germinal vesicle breakdown (the resumption of meiotic maturation). We plan to determine the characteristic changes in degradation of individual proteins synthesized during these times.